In a recorded data reproducing device, a jumping operation needs to be carried out so that a detecting point of recorded data on a recording track is consecutively jumped from a track currently being read to another aimed track.
FIG. 1 shows a block diagram of a prior art circuit which operates to perform such a jumping operation. In the figure, reference numeral 1 denotes a part of a recording track. When the center of a data detecting light spot 2 is on focused on the center of the track, the centers of the tracking light spots 3 and 4 are positioned on both side edges of the track. Accordingly, if the center of the data detecting light spot 2 is displaced from the center of the track 1, a difference occurs in the light received from the tracking light spots 3 and 4. The polarity of the difference thereof is varied according to an amount of displacement and its direction.
A tracking servo signal (B) is obtained from the output of a differential amplifier 7 the inputs of which correspond to the light received from the light spots 3 and 4. The servo signal (B) is applied through a loop switch 8 to an equalizer amplifier (EQ) 9 where the servo signal is subjected to phase compensation. The compensated signal is then applied through an adder 10 to an input of a driving amplifier 11. In response to the output of the driving amplifier 11, a driving coil 13 is energized to actuate a tracking mirror 12 to rotate in a direction orthogonal to the recording track 1. In this manner, the data detecting light spot 2 is controlled to track the recording track accurately during reproduction. In this manner a servo loop is operated.
The servo signal (B) is also applied to a waveform shaping circuit 14 where the signal is subjected to waveform shaping to produce pulse signals (C). The signals (C) are in turn supplied to a forward/reverse switching circuit 15. The switching circuit 15 operates to invert the phase of the pulse signals (C) in response to a forward/reverse reproduction instruction supplied from a forward/reverse instruction circuit 19. Reversal of the phase of the pulse signals is needed for the following reason. Since the phase of the servo signal is different by 180.degree. between the forward and reverse directions of the movement of the light spot, it is necessary to coincide the phase of the signals at both when the light spot is shifted to the right and to the left relative to the track. The output pulses of the forward/reverse switching circuit 15 are applied as clock inputs (CK) to an N-th counter 16. This counter starts counting in response to a jump instruction signal supplied from an operation unit 18.
The counter is constructed so that the counting control is effectuated by an external control signal and the number to be counted can be thereby varied. A high level signal (D) is generated by the counter 16 continuously for the period beginning when the counter is set until N pulses are received and counted. In response to the signal (D), the loop switch 8 is controlled and a jump signal generating circuit 17 is enabled to produce a jump signal. This jump signal is inputted to the adder 10 to energize the mirror drive coil 13. The application of a signal from the forward/reverse instruction signal generator 19 to the jump signal producing circuit 17 causes the jump signal to be generated in accordance with the polarity corresponding to the forward and reverse directions.
FIGS. 2(A) through 2(D) show waveforms of the signals (A) through (D) as indicated in FIG. 1 corresponding to the initial condition of the tracking servo loop switch 8 being in the closed condition and the circuit reproducing while the tracking servo is normally operated. In this condition, if the jump instruction signal as indicated in FIG. 2(A) is received from the operation unit 18, the N-th counter 16 is reset in response thereto and the counter output (D) is raised to a high level as shown in FIG. 2(D). This opens the loop switch 8. The jump signal containing a polarity instruction corresponding to the reproducing direction of forward or reverse is outputted from the jump signal generating circuit 17. By this jump signal, the tracking mirror 12 is forcibly rotated so that the data detecting light spot is caused to jump onto an adjacent track from the current track.
As a result, the output of the differential amplifier 7 produces signals as shown in FIG. 2(B), and the waveform shaping circuit 14 outputs pulse signals as shown in FIG. 2(C). The latter pulse signals are fed to the clock input CK of the N-th counter 16 and the number of pulses fed thereto are counted. Upon completion of counting N pulses, the output (D) of the counter 16 goes to a low level, thereby closing the servo loop switch 8 and ceasing the production of the drive signal. Thereafter, the reproducing operation is initiated.
Counting N pulses by the counter 16 indicates that the light spot 2 has jumped N tracks at once. Therefore, by effecting the count control in accordance with the external instruction, jumping of a desired number of tracks can again be carried out.
The above-described prior art circuit arrangement is disadvantageous in that, if the data detecting point does not accurately jump onto the aimed recording track, an over-run phenomenon occurs which is not detected. Therefore, the data detecting point cannot accurately track the aimed track.